Historically, local area computer networks (LANS) using optical data links have relied on light emitting diode (LED) sources launching into multimode optical fibers. The EIA/TIA and IEC Building Wiring Standards (TIA 568A) specified the use of 62.5/125 micron multimode optical fiber for intra-building wiring. These standards have resulted in the large-scale deployment of multimode optical fiber in existing computer networks.
In prior communication application technologies, these data transmission platforms have provided adequate bandwidth. Asynchronous transfer mode (ATM) computer networks can support data transmission rates as high as 622 megabits/sec (MBPS), but LED rise times, the chromatic dispersion associated with the relatively wide bandwidth of light produced by the LEDs, and multiple fiber transmission modes impose an upper cap on the potential data rates. Thus, LED/multimode fiber systems are generally limited to sub-gigabit/second (GBPS) data rates.
Newer computer applications requiring higher bandwidths and the increasing number of users that must be serviced by individual networks have led the push to provide GBPS performance, and better. In order to attain this performance in the context of existing optical data links, the LED light sources have been replaced with laser sources such as vertical cavity surface emitting lasers (VCSEL) and Fabry-Perot lasers. These devices can produce the necessary rise times and have the narrow spectral widths required for GBPS data transmission speeds.
Computer network links modified to use single mode laser sources, however, many times still fail to achieve the data/error rates at GBPS data rates that would be predicted solely from the laser source performance. The problem has been traced to computer links using multimode optical fiber. In many instances, a pulse-splitting phenomena is detected, which increases the bit error rates to unacceptably high levels at these speeds.
The obvious solution to this problem is to use single mode fiber with the single mode sources. While being viable for newly installed computer networks, such a solution is impractical for the installed base of multimode fiber networks since running new fibers in and between buildings represents a significant expense.
Other solutions have been proposed to constrain pulse splitting in signals from single mode sources that have been launched into multimode fibers. In one case, the signal from the single mode source is launched into a short-length pigtail of single mode fiber. The other end of this fiber is then coupled to the existing multimode fiber, offset from the multimode fiber core center.
The problem with the off-axis, single-mode/multimode fiber coupling solution is the difficulty to implement it in the typical computer network environment. The single mode fiber must be precisely misaligned to the multimode fiber such that the light is still launched into the multimode fiber with acceptable efficiency, and this misalignment must be maintained in the coupling module across its lifetime.
According to the present invention, pulse splitting is constrained in single mode source/multimode fiber systems by preventing light from entering the center of the multimode fiber by controlling the light emission characteristics of the laser source. Specifically, the laser source beam is annularly cross-sectioned. Thus, when properly aligned with the multimode fiber, little or no light is launched down the fiber""s center, avoiding the pulse splitting problem.
In specific embodiments, the center region of the annular beam has an optical power density that is reduced by 90% or more with respect to the beam. Further, the beam is preferably completely annular, i.e., forms a complete ring.
In other aspects of the embodiment, the laser source is a vertical cavity surface emitting laser. The annular beam is formed by a totally reflecting center block or stop in the VCSEL""s output aperture. The block is dimensioned to form a dark center region of approximately 5 to 7 microns in diameter in the resulting beam.
In general, according to another aspect, the invention features a method for improving modal bandwidth in a computer network. According to the method, a substantially annularly cross-sectioned beam is generated with a laser source. The beam is then coupled into the multimode optical fiber of a computer network for transmission to a detector.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.